Trans fats are a four letter word in nutrition circles — the boogeyman that corrupts your baked goods and tasty fried foods. But what makes these nutritional fats different from all the others? And why are they the bane of medicine?

Trans fats differ from other unsaturated fats from a structural perspective, but only a subtle one. A slight change in chemistry makes these tiny molecules a big problem for your body.

A tiny, tiny chemical difference
Fats are made up of long chains of carbons bonded together, with a acidic element at the end (this is why fats are often called fatty acids). There are two main types of fats - unsaturated and saturated. Unsaturated fats are molecules of fatty acids with a carbon-carbon double bond somewhere along the molecule. The other type of fat, saturated fat, lacks any double bonds — every carbon in the chain is "saturated" with hydrogen atoms. From now on, we will only be talking about unsaturated fats, as trans fats originate from unsaturated fats.

From a chemistry perspective, each double-bonded carbon within an unsaturated fatty acid chain is bonded to three other atoms. The carbon atom is bonded (through a double bond) to the carbon next to it, singly bonded to the carbon behind it in the chain, and bonded to a hydrogen.

It is the position of the hydrogen relative to the rest of the chain that makes all the difference between a trans fat and a regular cis unsaturated fat.

In cis unsaturated fats, the hydrogen is bound in the same orientation as the hydrogen on the other end of the carbon-carbon double bond. This orientation causes the hydrogen to be repelled by other the hydrogen across from it. This repelling due to an overlap of electron clouds causes a kink at the site of the double bond, with the other end of the fatty acid bending in a different direction.

However, if the hydrogen bonds at the opposite position — away from the adjacent hydrogen — the molecule of fat remains linear, creating a trans fat.

While both of these molecules of fat have the an identical chemical formula (same number of carbons, oxygens, and hydrogens), this little change is enough to make their geometric structures and chemical properties vary a great deal. And this is where the problems begin.

Trans fats and stacking
Trans fats, due to relatively linear shape when compared to other unsaturated fats, are able to bundle together and stack. This stacking property is key to the negative impacts of trans fats on health, as stacked trans fats are harder for the body to metabolize into useable energy.

Stacked trans fats are extremely stable and have much higher melting point than the corresponding cis unsaturated fat. This difference in melting point can be extreme — often upwards of 30 °C higher, making the difference between a solid or liquid at biological temperatures. The more stable trans structure causes the body a number of problems in breaking down the molecules.

Making trans fats
Trans fats can occur naturally, but most are created by adding hydrogens to vegetable oils (liquids fats) to cheaply create a solid fat that is stable at room temperature. This process creates a form of fat that is cheaper and easier to acquire than animal fat.

By heating liquid vegetable oils with multiple double bonds in a high pressure environment in the presence of nickel, hydrogens are added to the individual fatty acid molecules. As these hydrogens are added, a high percentage of these hydrogens add in the trans position, leading to an inordinate amount of trans fats in processed foods.

This hydrogenation process is commonly used to turn linoleic acid, an unsaturated component of corn and soybean oil featuring two carbon-carbon double bonds, into margarine.

Health problems associated with trans fats
The National Academy of Sciences declares that there is no safe level for consumption of trans fats, stating that ingestion of any amount of these molecules increases the probability of heart disease. Occurrences and deaths due to coronary heart disease, a narrowing of blood vessels moving oxygen and blood to the heart, is linked to diets high in trans fats.

When calorie counts are equal, monkeys fed foods high in trans fats gained additional abdominal fat and weight gain compared to monkeys fed a low trans fat diet over a six year study. Whether this correlation extends to humans is not definitively known, as long term studies of humans maintaining equal calorie counts are a difficult undertaking.

Trans fats are more difficult for the liver to process than other fats, as trans fats interfere with liver enzymes. Trans fats also pose a problem to lipases, enzymes that break up fats. Lipases are capable of breaking down cis fatty acids, but the enzymes are unable break down trans fatty acids with the same efficiently.

An increase in depression?
One of the more interesting negative effects of eating foods high in trans fat comes in an unforeseen neurological effect - an increase in depression. In a study conducted in Spain, patients without a history of depression who consumed diets high in trans fats experienced a nearly 50% increase in major depressive disorder.

A number of factors could link the eating of fried and processed foods to depression, particularly the decreased cost of these foods and economic status. However, a biochemical theory explaining the increase in depression calls for the substitution of edible trans fats for a similarly structured molecule in the orbitofrontal cortex, a portion of the brain governing emotional response to external rewards.

Researchers have looked for a correlation between the consumption of trans fats and a cancers. Despite their efforts, no definitive link between the two has been established.

Top image is from Paul_Brighton via Shutterstock. Image of cis and trans-oleic acid as well are within the public domain, as well as the space-filling structuralimages of unsaturated fatty acids used to show the difference in geometry. Sources linked within the article.